Time Assignment Speech Interpolation (TASI) ("Overall Characteristics of a TASI System," J. M. Frazier, D. B. Bullock, and N. G. Long, BST J, July 1962, and "TASI Quality-Effect of Speech Detectors and Interpolation," H. Miedema and M. G. Schactman, BST J, July 1962) was the first telephone system to exploit the idle time in a telephone call. The basic idea was for a number of calls to share a lesser number of channels. High speed switching connects a speaker to a channel when a talkspurt is detected. When the talkspurt ends that channel is available for use by another active speaker. This system can approximately double the call capacity of telephone channels.
When the number of simultaneous speakers exceeds the number of channels some speech is lost. This loss of speech is called freeze-out. The amount of speech lost must be kept small so as to not appreciably affect quality. Speech loss on TASI generally averages less than 0.5 percent.
TASI was designed to give connection priority to already connected talkspurts. When a call became active it was connected to an available channel. If none were available, the new talkspurt was frozen out until a channel became available.
Digital implementation of TASI ("Digital Speech Interpolation," S. J. Campanella, Comsat Technical Review, Spring 1976) showed improvements over the original analog system. Incoming calls were digitized into 7 bit-per-sample pulse code modulation (PCM) at 8K samples per second. The freeze-out criterion was different from the original TASI. Instead of using percent of speech frozen out, the criterion was to hold freeze-outs longer than 50 msec to less than 2 percent. This was considered preferable to a fixed percent frozen-out since it related more closely to the cause of degradation, initial talkspurt freezeout.
In a further improvement, speech is digitized at 32K bits-per-second by syllabic companded delta modulation (SCDM). FIG. 1 schematically shows an SCDM circuit 10, an example of which is described in copending U.S. Pat. No. 4,208,740, Ser. No. 971,587, filed Dec. 20, 1978 by R. J. Hallett, et al., assigned to the instant assignee.
A comparison is made by comparator 2, at the clock rate, between the analog input and the analog estimate. The digital output represents the comparison results. If the estimate is less than the input, a positive increment equal to the step size is presented by flip-flop 4 to the integrator 6. An estimate larger than the input results in a negative increment. With companding by the compander 8, the step size is a function of comparison history. For example, if contiguous samples show the estimate too small, the step size increases. Alternating comparison results indicate good tracking, resulting in a constant step size. Good signal-to-noise performance can be achieved over much wider input signal range with companding ("Delta Modulation," H. R. Schindler, IEEE Spectrum, October 1970).
In an example time division multiple access (TDMA) satellite communication system, the system frame time is 15 msec. That is, packets of speech representing 15 msec, digitized into 480 bits, are transmitted to the receiver every 15 msec. FIG. 2 shows the transmission system. For example, from 100 calls shown in progress, the digital multiplexer 12 selects and attempts to fill the fifty 480 bit slots in the high speed transmission channel 14. A freeze-out occurs in call 99 because it happened to be the 51st packet selected and all available channels (50) have been used, as shown in the TDMA frame of FIG. 3. The demultiplexer 16, receiving no transmission for call 99, inserts idle noise from source 18 for that 15 msec interval.
The presence of speech at the transmit end is determined by a speech detector for example, with a fixed threshold of about -33 dBmO. Only frames with speech or overhang are candidates to be transmitted. Overhang refers to a number of frames which are to be transmitted following any frame with activity. The presence of overhang tends to reduce choppiness introduced by threshold levels.
The two TASI systems mentioned have a similar characteristic. That is, if a talkspurt suffers initial freeze-out, the next transmission received gives the actual value of the waveform. SCDM is somewhat different however. Freeze-out causes the receiver to lose step size information. When transmission resumes, the receiver might not return to the correct signal level during that talkspurt. This characteristic of SCDM creates a different level of acceptable freeze-out criteria than those for TASI systems.
Freeze-out of 15 msec duration in the body of talkspurts (approximately 100 msec from the start) are almost always noticeable. The audible effect is like a bubble in the word and word meaning is sometimes blurred.